The present invention relates in general to telephone circuits, and more particularly to a circuit for detecting grounding and short-circuiting in a subscriber line interface circuit employed in a private branch exchange (PBX) and the like.
Grounding refers to a state in which a battery lead-in wire (a -48V wire, hereinafter referred to as wire A) of two feeders (a battery lead-in wire A and a ground wire B) which connect a subscriber terminal (a telephone set) with a telephone exchange is short-circuited to ground due to a fault.
A "short-circuit" to a situation where a ground wire (hereinafter referred to as wire B) is connected to a power source (-48V) for some reason (disconnection of the wire A and consequent short-circuit thereof with a wire B for instance). In the case where these states take place, they must be detected to stop the supply of current to the telephone set.
Regarding the circuit for detecting grounding, a circuit has been described in the Japanese Patent Publication No. 186/1984, which circuit is characterized in that it is suitable circuit integration and enables the detection of a ground condition by use a technique of loop current detection. However, detailed examination of this circuit by the present inventor has made it clear that the occurrence of an abnormality can not be detected even by said disclosed loop current detection technique in at least two cases.
(1) A constant-resistance feeding system (adopted in Japan) and a constant-current feeding system (adopted in U.S.A. and other countries) are used as feeding systems from a telephone exchange to a telephone set. According to the aforesaid disclosed technique, a mode of operation is possible wherein a ground can not be detected in the constant-current feeding system.
(2) Simultaneous occurrence of the ground and the "short-circuit" conditions can not be detected, even when the sole occurrence of either of them can be detected.
The above-mentioned two problems are essential for understanding the effect of the present invention, and therefore they will be described in more detail hereunder with reference to the drawings.
(i) Definitions of the constant-resistance feeding system and the constant-current feeding system
The constant-resistance feeding system is a system for feeding a current from a power source (-48V) having a constant internal impedance R.sub.E, as shown in FIG. 4(a). In the figure, R.sub.L denotes the line impedance.
The constant-current feeding system is a system for feeding a constant current irrespective of the value of the line impedance R.sub.L, as shown in FIG. 4 (b).
FIG. 5 (a) is a circuit diagram of a simplified circuit based on the circuit described in the foregoing Japanese Patent Publication No. 186/1984 and applied to a constant-current feeding system. In this diagram a comparator circuit 12 is provided for comparing the respective output signals of sense circuits 10 and 11. In this circuit, an imbalance occurring between the currents flowing through the wires A and B when a ground condition occurs is so utilized as to generate an abnormality detection signal when the difference between these currents reaches a prescribed value or above. In the constant-current feeding system, however, the currents flowing through sense resistors R.sub.S and R.sub.S ' do not change and consequently no abnormality can be detected even when the wire A is "short-circuited" while busy, as shown in the figure.
(ii) Problem of simultaneous occurrence of "short-circuit" and ground conditions
FIG. 5 (b) also shows a simplified version of the aforesaid described circuit. When the wire A is grounded and simultaneously the wire B is "short-circuited" as shown by dotted lines in this figure, abnormal currents I.sub.E and I.sub.E ' flow through the sense resistors R.sub.S and R.sub.S ' in the same way, the imbalance between the currents does not occur, and thus no abnormality can be detected.